Rathke, Growth of the Eggs of the Crawfish. 255 
volution of the sacculus in one direction only. The result of this dis- 
tinction is, that the central parts of the nervous system are in Vertebrata 
external, and in Annulosa internal, to the serous membrane. The 
positions of the ganglionic cord corresponding with the spinal marrow, 
and of the intestinal nerve corresponding with the great sympathetic, are 
completely reversed in these two great divisions of the animal kingdom. 
A similar opposition occurs in the general position and direction of all 
the external organs. Inthe Vertebrata, in Batrachia for instance, both 
extremities, the head and tail, as soon as they become visible, expand over 
the vitellus, and tend to inclose it. In the same manner, the legs and 
arms, and in Birds the wings, embrace the vitellus, which is placed in 
juxtaposition with the abdominal surface of the body; the primitive 
portion of the embryo remaining at the back. In the Crawfish on the 
contrary, the tail tends to become free, while the anterior extremity alone 
is applied to the vitellus, the two ends approaching each other not by 
surrounding the last named organ, but in the contrary direction. The 
limbs too, instead of embracing the vitellus, take the opposite direction 
and surround the outerside of the primitive portion of the embryo. 
Hence it follows that in the Crawfish, and the same is the case in Spiders, 
the inner side of the limbs corresponds with their outer side in Vertebrata ; 
and that, when the Crawfish quits the egg, it turns the primitive part of its 
embryo towards the earth, while the Vertebrata turn their’sin the opposite 
direction. What iscalled the abdominal surface in the former is conse- 
quently analogous to that which is termed dorsal in the latter; and 
Annulosa turn their backs, while Vertebrata turn their bellies, towards 
the surface of the earth. 
Other important points of distinction are found in the structure and 
developement of the head and its parts, and in the disposition of the 
brain and nervous system in general. But we cannot afford space to enter 
further into these particulars. The authour concludes with some general 
deductions, which ate, however, sufficiently obvious not to require 
repetition. Three plates, two of them accompanied by outlines, filled with 
magnified representations of the egg and its contents during the various 
stages of developement, and in various points of view, together with 
their explanation at length, complete the volume. They are executed 
with great care, and afford excellent illustrations of the growth of the 
embryo in all its stages. 
